I stole this from GameFaqs:
"Yep. The voided pool works like this.
The villagers on main street will stay there even if they are no longer in your pool. That information is stored in two different places. Just because you see a villager on main street doesn't mean they're in your voided villagers pool that can be passed to other people. That's just a way for you to see who you've moved out of your village.
The villagers that go to the void will stay there until you either streetpass with someone or go on wifi with someone, and that's anyone. If you go on wifi with somebody or streetpass and they have ten villagers, it clears your entire void and all of them are just gone.
If you wifi or streetpass with someone that has less than ten that person will get one of the voided villagers, and if you had any others, they all just disappear from your void. Your friend could have gotten any of those 25 villagers if none of them were adopted by someone else, they just so happened to get the second one that you moved out."
